The present disclosure relates to carbon compositions suitable for coatings, particularly coatings for moving parts exposed to harsh environments.
Fuel delivery systems include various moving components that operate at relatively high temperatures and at relatively high pressures. Fuel delivery systems may include rollers, shoes, tappets and the like, which are exposed to vibratory and oscillatory motions. Sliding tappets may be prone to scuffing between the tappet and a corresponding shaft as the tappet alternates direction. Direct metal-to-metal contact of rolling or sliding elements within a diesel or other fuel delivery system may cause false-brinelling, wear, fretting, microploughing, microcutting, scuffing, galling, and eventually failure of the component itself.
Materials comprising sulfur, nitrogen, and phosphorous naturally found in, and/or added to fuels, for example diesel fuel, provide lubricity. However, sulfur, nitrogen, and phosphorous are being removed from fuels to address various environmental and other issues. As such, the lubricity of fuels, in particular the lubricity of diesel fuel, is decreasing. As the lubricity decreases, the scuffing and galling of fuel delivery components increases.
Coating various components of fuel delivery systems with wear resistant coatings is one approach directed to reducing wear. However, coatings such as silicon carbide (SiC), titanium nitride (TiN), titanium carbide (TiC), chromium nitride (CrN), and the like, may not provide for components that meet modern day durability requirements. The hardness and relatively high friction coefficients of carbides and nitrides may cause abrasive wear to occur. Also, the relatively high temperatures needed for carbiding and nitriding can reduce the hardness and fatigue properties of the various components coated therewith.
Tungsten carbide and carbon coatings (WC/C) may be adequate for some surfaces within a fuel delivery system, especially those surfaces having relatively low contact stress including sliding surfaces. However, such coatings may not provide long durability for rolling and scuffing applications where the components are subjected to strong rolling and/or oscillatory sliding motions.
Accordingly, a coating that may be employed to protect various high wear components such as those operational within a fuel delivery system would be beneficial.